Depending upon the use to which the coating is to be put or the substrate on which the coating is put, the temperature at which a coating can be deposited may be important. Critical temperatures are often involved, above which the properties of the substrate are altered.
For coating drills and machine tools with compounds of the titanium sub-group of elements to lengthen the useful life of the tool, nitrides, carbides and carbonitrides of titanium metal are desirable coatings. Compounds, such as titanium nitride, can be deposited as coatings by chemical vapor deposition (CVD) at temperatures of about 1000.degree. C. or higher.
However, most tool steels are hardened and tempered; and, if temperatures exceeding the tempering temperatures are used, the tools lose their temper and must be reheat treated. When reheat treated, distortion becomes a problem and tool tolerance are affected. Thus, it is preferable to carry out any coating deposition at temperatures below the softening point of the tool metal in order to avoid affecting the temper of the tool steel.
The coating of metalworking tools with titanium nitride is a typical example of a coating situation in which low temperature deposition of the coating is desirable. Present processes deposit titanium nitride by chemical vapor deposition by passing titanium tetrachloride, nitrogen and hydrogen over the substrate at temperatures of about 1000.degree. C. or higher. Other existing processes, also operating in the range of 1000.degree. C., add a hydrocarbon gas, such as methane, to include titanium carbide in the coating deposit. These temperatures have detrimental effects on tool steel temper. Reheat treating to reestablish temper detrimentally affects tolerances. For example, tempered steel will soften about 500.degree. to 600.degree. C., stainless steel will sensitize above 550.degree. C. and nickel-based super alloys overage above 600.degree. C.
It has been found that low deposition temperatures below the temper temperatures of such metals can be used to deposit metal compound coatings of the type discussed above by utilizing sub-halide compounds of the metal in question in the deposition reaction. The phrase metal sub-halides refers to metal halides in which the metal exhibits a valency state (oxidation state) lower than the maximum valency state known for that metal. For example, the maximum valency for titanium is 4, that is, TiCl.sub.4. Therefore, the halide compounds of TiCl.sub.3 and TiCl.sub.2, where titanium exhibits a valency of 3 and 2, respectively, are titanium sub-halides and, in particular, titanium sub-chlorides. Similarly, where the maximum valency state of the metal is 5, any halide compound of that metal, wherein the metal exhibits a valency lower than 5, would be a sub-halide of that metal.
In Canadian Pat. No. 1,087,041, the deposition of hafnium carbide and hafnium nitride coatings by chemical vapor deposition is discussed. At that time, temperatures typically in the neighborhood of 1300.degree. C. were required to produce hafnium coatings by chemical vapor deposition. That patent discloses the use of the sub-halides of hafnium in the deposition process, and thereby achieves temperatures as low as 900.degree. C. in the deposition reaction. However, as discussed above, 900.degree. C. is still above the temper loss point for many alloys that it is desirable to coat.
Low pressure CVD is disclosed by Barron et al, Vacuum Systems for Plasma Etching, Plasma Deposition and Low Pressure CVD, Solid State Technology (September 1978) and Jensen, Modelling of Chemical Vapor Deposition Reactors, 9th International Conference in Chemical Vapor Definition (May 7-10, 1984), which are hereby incorporated by reference.